Since microwave ovens were first introduced, it has been well known that the spatial distribution of microwave energy in the cavity tends to be nonuniform. The nonuniformity has resulted in hot and cold spots within food being cooked and this, of course, is undesirable. Accordingly, there has been extensive and continuous development work to improve the time averaged spatial distribution of energy within the cavities of microwave ovens. The spatial distribution is in part a function of the reflections of microwave energy from the conductive walls thereby producing complex configurations of electromagnetic fields which have commonly been referred to as modes. Simply stated, a major reason for the nonuniformity of the spatial distribution of microwave energy is the constructive and destructive interference of reflections. For example, where reflections add, a hot spot is created and where they subtract, a cold spot is created.
The most common approach for improving nonuniform heating in prior art microwave ovens has been to use a mode stirrer which attempts to randomize reflections and alter the modes by introducing a time-varying scattering of the microwave energy. Typically, a mode stirrer is a metal paddle or propeller which rotates to alter the modes so that the spatial positions of constructive and destructive interference move.
Mode stirrers have been rotated using either a motor drive or directing a flow of air against the propellers. In one air flow approach, a vertical microwave transparent posts extends upward from a mounting bracket and an axial bore in a microwave transparent hub of the metal mode stirrer inserts over it. A small low-friction ball was positioned on top of the post to minimize the rotational friction between the hub of the mode stirrer and the post.
Another prior art approach to providing uniform spatial distribution of microwave energy was to utilize a primary radiator. More specifically, the microwave energy is coupled directly from the waveguide to a microwave antenna which preferably has a directive pattern. Further, uniformity has been provided by rotating the antennas. One prior art approach for supporting a rotatable microwave antenna and coupling microwave energy to it is described in U.S. Pat. No. 4,335,289. An antenna probe, which extends through an aperture between the waveguide and the cavity, is excited and functions as a coaxial center conductor. The microwave antenna was connected to the probe in the cavity. To rotatably position the antenna probe in central alignment within the aperture, a plastic bushing with a central bore was mounted in the aperture and the antenna probe was inserted therethrough. A flow of air was blown against vanes connected to the antenna to provide rotation. Although the bushing adequately functioned to align vertically the antenna probe in the center of the aperture, the friction between the probe and the bushing caused the antenna to rotate at a relatively slow speed. This was a problem becuase the scan rate of a factory leakage test is specified by an approval agency to be a function of the antenna rotation speed. Accordingly, if the antenna is rotating slowly, each oven has to be tested for a longer period of time. The problem would become exaggerated if the air flow was reduced for any reason.